Various techniques for depressurizing high pressure gas streams from transport mode to end user mode are known in the prior art. In some instances, high pressure gas is merely throttled through a Joule-Thompson valve to reduce its pressure without the recovery of the energy of compression. This results in a low pressure relatively cold product stream wherein no energy recovery is accomplished. It has also been known to depressurize high pressure gases for low pressure end use by expanding high pressure gas through a work turbine or a Joule-Thompson valve with the subsequent reheat of the cooled low pressure gas product by various non-renewable energy sources. These energy sources include a portion of the high pressure gas, in the instance of combustible gas such as natural gas, or other fuel sources readily available at the site of depressurization, such as coal or fuel oil. In some instances, rewarming mediums are readily available if the depressurization site is near large quantities of warm water, such as sea water, or the site is adjacent other industrial plants wherein waste heat in the form of steam is available. In those latter instances, reheat of the cooled depressurized gas stream is performed against such available warmer temperature mediums which allow the depressurized gas to be maintained at relatively warm temperature conditions.
Rewarming of depressurized gases is typically necessary in order to avoid problems arising from low temperature conditions which would otherwise result from depressurized gas. The problem is particularly acute wherein energy recovery from the depressurization stage is desired. Where energy recovery is not desired and high pressure gas is expanded through a Joule-Thompson Valve without partial condensation, a relatively small cooling effect is accomplished and such colder temperatures are not detrimental to eventual end use of the depressurized gas. However, where energy recovery is desired and high efficiency of energy recovery is achieved, the depressurized gas typically attains very low output temperatures from the expansion means, such as a turbo expander. These low temperature conditions, wherein energy is efficiently recovered in the depressurization, result in undesirable temperature-induced conditions downstream of the depressurization, including; frosting of the conveyance equipment, equipment materials problems from the stress of low temperatures, condensables, hydrate and ice formation in the gas and safety hazards with regard to extremely low temperature conditions being present near operating personnel.
In recent years, governmental regulations have encouraged the use of renewable energy sources for various industrial processes through tax incentives and other incentives. With regard to power recovery systems which require classification as small power plants, government regulation has required that only 25% of the total available energy utilized in such small power plant may come from non-renewable energy sources. Such requirements are particularly pertinent to power plants operating to recover power from natural gas being depressurized from pipeline pressures to end user pressures, wherein the reheating necessary to return the depressurized natural gas to relatively warm temperatures has traditionally been performed by utilizing a portion of the natural gas fired in a natural gas burner. Such a reheat mode would not comply with present-day government standards for classification as a small power plant.
Various systems are known for recovering power from natural gas including U.S. Pat. No. 4,400,947 which utilizes rewarming liquefied natural gas to liquefy a refrigerant which is pumped to high pressure before being expanded with power recovery to low pressure for reliquefaction. The natural gas is at low pressure at all times in that facility.
It is also known to expand natural gas to low pressure for end use in an expansion turbine wherein the power is utilized to compress already high pressure natural gas to further elevated pressures necessary to liquefy a slipstream of the natural gas for storage during off-peak natural gas consumption time periods, while achieving the depressurization of natural gas for present consumption. Such a process is shown in an article "Largest Expander Cycle LNG Plant Now In Operation" by Irving Weiss and Steven J. Markbreiter appearing in Cryogenic Engineering News, March 1969, pages 24, 25, 40 and 44. This type of plant is called a peak-shaving plant, wherein at low consumption times, such as summer time, natural gas is depressurized and the cold temperatures resulting from depressurization are utilized to liquefy a portion of the natural gas for storage and later use during high consumption times, such as winter time. In such a cycle, the low pressure natural gas is rewarmed against liquefying natural gas slipstream, and the heat of compression of the feed natural gas is recovered in the rewarming natural gas for distribution. Such a cycle is shown in U.S. Pat. No. 3,360,944.
Both of these prior art cycles are involved in supply-end power recoveries and reheat provisions, rather then end user-located depressurizations and reheating circumstances. The present invention constitutes an improved method for depressurizing high pressure pipeline natural gas to low pressure end user conditions, wherein the low pressure gas is maintained at a relatively warm temperature by reheating with renewable energy and the energy of depressurization under conditions so efficient that net power is recovered. Contrary to the drawbacks of prior art systems which require an available heat source such as waste, industrial heat or sea water, the present invention may be utilized at remote sites where no readily available heat source is convenient or at sites where waste heat utilization is uneconomical.